Due to their high storage density, long data retention life, and relatively low cost, optical disks have become the predominant media format for distributing information. Large format disks, and more recently, DVD disks, have been developed for storing full length motion pictures. The compact disk (CD) format was developed and marketed for the distribution of musical recordings and has replaced vinyl records. High-capacity, read-only data storage media, such as CD-ROM and DVD-ROM, have become prevalent in the personal computer field, and the DVD format may soon replace videotape as the distribution medium of choice for video information.
Recently, relatively inexpensive optical disk writers and writable optical media have become available, making optical disks popular as backup and archival storage devices for personal computers. The large storage capacity of writable optical disks also makes them ideal for use in multimedia authoring and in other applications that require access to large amounts of storage. Current writable optical disk technologies include several write-once technologies, such as CD-Recordable (CD-R) and DVD-Recordable (DVD-R); a few technologies permit writing, erasing, and rewriting data on a disk, such as Mini-Disk (MD), which uses magneto-optical technology; still others use phase-change and dye-polymer technology. Recent advances in writable optical disk technology have made rewritable optical media more practical, and the specification for DVD-RAM calls for use of high-capacity rewritable optical media.
An optical disk is made of a transparent disk or substrate in which data, in the form of a serial bit-stream, are encoded as a series of pits in a reflective surface within the disk. The pits are arranged along a spiral or circular track. Data are read from the optical disk by focusing a low power laser beam onto a track on the disk and detecting the light reflected from the surface of the disk. By rotating the optical disk, the light reflected from the surface of the disk is modulated by the pattern of the pits rotating into and out of the field of laser illumination. Optical and imaging systems detect the modulated, reflected, laser light and produce an electrical signal that is decoded to recover the digital data stored on the optical disk.
Data is typically recorded on writable optical disks by using a higher power laser than is used for reading. The media for use with optical disk writers typically includes a recording layer, made of a material that changes its optical characteristics in response to the presence of the beam from the high power laser. The high power laser is used to create "pits" in the recording layer that have a different reflectivity than surrounding areas of the disk, and that can be read using a lower power reading beam. In systems having the ability to erase and re-record data, a laser having a power output between the low power used for reading and the high power used for writing may be used to erase data. Alternatively, some systems employ a laser that outputs a different wavelength of light to erase data from the optical media. The methods used to write and erase optical disks depend on the type of recordable media being used.
To write or retrieve data from an optical disk, the foregoing optical systems include a pickup assembly that may be positioned to read or write data on any disk track. Servo mechanisms are provided for focusing the optical system and for keeping the pickup assembly positioned over the track, despite disk warpage or eccentricity.
The automatic focus system used in an optical disk drive must be very sensitive. If the system is not able to properly focus light onto the surface of the disk, the phase interference between the light reflected from the pits and from the areas surrounding the pits may be lost, making the data unreadable. For writing, improper focus may cause the energy of the writing beam to be spread over too large an area to permit effective writing of the optical disk.
Even the most carefully manufactured disk is not perfectly flat, and even the best optical disk reader is unable to spin the disk at the required speeds of 200 RPM and higher with no variation in the vertical offset of the disk. The specifications for reading a compact disk (CD), for example, allow for variation in the vertical offset of the disk of .+-.600 microns, while the beam must remain focussed to within .+-.2 microns. It is therefore necessary to have a focus system that is able to keep the surface of the disk in focus as the vertical offset of the disk varies.
Focus systems generally used in optical disk readers measure certain parameters of the light spot formed by the illumination beam reflected from the optical disk. One previously known method of detecting focus errors in optical disk readers is the astigmatism method. In this method, a cylindrical lens is placed in the optical path of the system to introduce astigmatism into the reflected beam. The beam is then focussed onto a quadrant detector consisting of four equal-area photodetector segments.
When the beam is in focus, the image projected onto the detector is circular, with light falling equally on all four segments of the detector. When the beam is out of focus, the astigmatism introduced by the cylindrical lens causes the image projected onto the quadrant detector to become elliptical, so that two of the segments of the detector receive more light than the other two, depending on the direction and degree to which the system is out of focus. Signals from the segments of the quadrant detector are arithmetically combined to produce a focus error correction signal. That signal is in turn used to drive a servo that moves an objective lens toward or away from the surface of the optical disk to keep the disk in focus. More information on the astigmatism method, and other methods of detecting and correcting focus errors in optical disk readers may be found at pages 140-142 of H. Nakajima and H. Ogawa, Compact Disc Technology, (translated by C. Aschmann), published by Ohmsha, Ltd., Japan (1992), and at pages 111-117 of K. Pohlmann, The Compact Disc Handbook, (2nd ed. 1992), published by A-R Editions, Inc., Madison, Wis.
Because in most previously known systems the data are read from the disk serially, i.e. one bit at a time, the maximum data transfer rate for an optical disk reader is determined by the rate at which the pits pass by the pickup assembly. The linear density of the bits and the track pitch are fixed by the specification of the particular optical disk format. For example, CD disks employ a track pitch of 1.6 .mu.m, while DVD employs a track pitch only about one-half as wide.
Previously known methods of increasing the data transfer rate of optical disk readers and writers have focused on increasing the rate at which the pits pass by the pickup assembly by increasing the rotational speed of the disk itself. Currently, constant linear velocity (CLV) drives with rotational speeds of up to 16.times. standard speed are commercially available, and even faster reading speeds have been achieved using constant angular velocity designs. Higher disk rotational speeds, however, place increasing demands on the optical and mechanical subsystems within the optical disk player, create greater vibration, and may make such players more difficult and expensive to design and manufacture. Higher rotation speeds also make accurately writing data to a disk more difficult, so few CD-R systems are available that record at faster than 4.times. standard speed.
A cost effective alternative to increasing the disk rotational speed is to read multiple data tracks simultaneously, as described in commonly assigned U.S. Pat. No. 5,426,623 to Alon et al. In accordance with the methods and apparatus provided therein, for example, ten adjacent data tracks may be read simultaneously. Thus, even if the disk is rotated at only 4.times. the standard speed, the capability to read ten tracks simultaneously provides the equivalent of a 40.times. drive.
It should be noted that as used herein, a data track is a portion of the spiral data track of a typical optical compact disk that follows the spiral for one rotation of the disk. Thus, a drive capable of reading multiple data tracks simultaneously reads multiple such portions of the spiral data track at once. For optical disks having concentric circular tracks, a data track would refer to one such circular track. For disks having multiple concentric spiral tracks, a data track would refer to one of the concentric spiral tracks.
One way that a drive capable of reading and writing multiple data tracks simultaneously may be implemented is through use of multiple beams, arranged so that each beam illuminates a single data track on the disk. U.S. Pat. No. 5,144,616 to Yasukawa et al. shows a system in which multiple laser diode emitters are used to provide multiple beams. Other methods may also be used to provide multiple beams, though some of these methods may not be appropriate for use in writing multiple tracks simultaneously. U.S. Pat. No. 4,459, 690 to Corsover, for example, describes a multi-beam system in which an illumination beam generated by a single laser source is split into multiple beams using an acousto-optic device that dithers the beam in a direction normal to the track direction.
The beams in a multi-beam optical pickup may also be provided by using a diffractive element to split a single beam into multiple beams. This technique is used to generate the beams in a three-beam tracking system, as shown in The Compact Disc Handbook, Pohlmann, K., 2nd ed., A-R Editions, 1992, pp. 108-115. In commonly assigned, copending U.S. patent application Ser. No. 08/911,815, a diffractive element is used to split an illumination beam into a plurality of reading beams. Through careful design, it is possible to produce a diffractive element capable of generating multiple reading beams properly aligned with the data tracks of an optical disk.
Multi-beam systems, however, may cause difficulties for automatic focus systems. If the astigmatism method is used with a standard cylindrical lens, for example, the image of the spots projected onto the focus detector may have a relatively large diameter. While this is not a problem for a single beam system, in a multi-beam system, the spacing between the beams places severe constraints on the size of the focus detector. If the detector is too large, multiple spots will impinge in the detector. The large diameter of the spots in such a system may also cause crosstalk between neighboring beams in a multi-beam system.
Additionally, the elliptical spots projected when the system is out of focus may be much larger than the circular spots that are projected when the beam is in focus. Thus, for example, when multiple beams are used, the elliptical projections of the multiple beams may extend beyond the focus error -detector, multiple spots may impinge on the focus detector, or the spots may overlap each other, thus making it difficult to obtain a focus error signal that accurately measures the magnitude of the focus error.
It would therefore be desirable to provide a focus error detection system designed for use in a multi-beam optical pickup.
It also would be desirable to provide a focus detection system that accounts for overlap between the spots projected by a multi-beam optical pickup.